While this is a philosophy forum, the scientific method is the undisputed source of what we call truth in this world. You probably wouldn't be here if not for its ability to link ideas and ways of thinking to a correspondence in reality. These ideas allowed chemistry to create fertilizer to allow you to exist [w][ytb], but also factory farming.

Looking as psychology then (a field also grounded in the scientific method), It is extremely hard to deny the importance of cognitive dissonance when talking about how people think about eating meat. I'd argue that a lot of the arguments here against vegetarianism boil down to some level of cognitive dissonance. Why?

You believe:

• You have choices
• You should make good choices
• You should be reasonable
• Hurting people is wrong in and of itself

***Why does everybody believe these things? I don't know. Perhaps because cultures with these values were better at cooperating and sticking together over time, out-competing selfish (for example) ones in their evolution. (natural selection / game theory). At the least, those who disagree with one of these are probably a small minority.***

From wiki, most people use one of the following as justification:

• Appeals to human evolution or to carnivory in nature ("natural")
  
  → human ancestors were nearly all vegetarian
  → forced sex and eating children are "natural" in other animals
• Appeals to societal or historical norms ("normal")
  
  → alcohol is the most harmful drug in the world, and yet it's "normal"
  → cigarettes, too. I mean really, our whole lives are historically extremely strange so this argument makes no sense
• Appeals to nutritive or environmental necessity ("necessary")
  
  → I haven't seen that argument here, yet, probably because nothing supports it
• Appeals to the tastiness of meat ("nice")
  
  → This one seems the most honest, but this sensory pleasure must outweigh some pretty grim harms

One can see pain in animals, and the science certainly finds that animals can feel pain. We evolved from monkeys, so we are really not that different physiologically in terms of pain to a mammal like a pig or cow.

In terms of number of deaths, its billions per year. Are they humanely killed? Not really.

At this point, nobody acting in good faith and trying to be reasonable is going to rest their argument on some silly little idea like 'animals aren't people so their pain doesn't matter'. They just don't want to admit they are acting in opposition to their moral values: ie. cognitive dissonance. The real reasons lie in that psychology article on wikipedia.

There is no point in debating this: people decide according to their own inclination and every decision is defensible in some manner. — Vera Mont

But I think people's inclinations can be affected by arguments if they are a reasonable person, which I think most people are (or believe themselves to be). If people with differing beliefs share their ideas in a way designed to be relatively unbiased, weaker arguments will have larger holes poked in them, making it just a bit more untenable to hold them. This may tip one's inclination to another viewpoint.

There are obviously many limitations and ways that this process could fail: allowing one side to talk more, one side acting more confident / assertive. But on average, more truthful arguments receive some advantage from their truthfulness.

The cynic in me says that some topics, perhaps the one about vegetarianism, could be tipped in one direction by so many other factors (ie maybe one side has a bullhorn) to a degree where not even the truth will be enough to ensure it is more convincing. This is something I wonder: how much of what somebody believes is because it is true, and not because of environmental factors that led to a persuasive effect?

If I'm permitted to kill a chimpanzee, feast on its meat and a chimpanzee is only 0.01% different from us genetically, can a species 0.01% above us, DNA-wise, do the same to us? Perhaps there's some kind of threshold of intelligence beyond which predation is impermissible and we've defintely reached that point, perhaps progressed beyond it, oui mes amies?

As an aside, genetic similarity measured in percent is not a great measure of anything really meaningful. Because of the nature of comparing DNA, there will be 25% similarity between two random DNA sequences, and yet we aren't 25% similar to a dandelion in nature. Even measuring intelligence is relatively complicated and a relatively arbitrary measure, I would argue, so I don't think intelligence should factor into whether it is permissible or not. If a shark kills another animal, obviously you can't do much about it, but that doesn't mean that it should be permissible.

And to the other comments asking for a reason why not to eat meat: obviously it hurts animals. The benefit, which seems to be just sensory pleasure, doesn't justify that to me.

Even if you have different values, it's hard to value human well-being and draw the line at animals. You certainly can draw that line, but it seems arbitrary.